一系列含苯咪唑配位基之異種釕金屬錯合物之設計與合成及其在染料敏化太陽能電池上的應用

Abstract

本論文目的為發展一系列新型態的釕金屬染料於染料敏化太陽能電池之應用。我們設計了一系列含苯咪唑配位基之異種釕金屬錯合物(RD系列染料)。為了與一般常見以聯吡啶為配位基的高效率染料 (N719，Z907，C101)有所區分。首先，我們設計了含有苯咪唑配位基之異種釕金屬錯合物。苯咪唑，由於它的電子傳遞能力很好，而且它的一些衍生物被廣乏的應用在有機發光二極體(OLED)領域上，用來當作電子傳遞及電洞攔截材料。RD1，我們所設計的第一個含苯咪唑配位基的染料，由於它的高染料吸附量，使它的效率能夠達到著名N719染料的八成。接下來，我們設計了一系列於苯咪唑環上不同取代基的染料，最後以修飾上甲基苯環的RD5光電轉換效率最高，達到7.7%，N719效率為7.8%。與N719染料比較起來，RD系列染料有一些缺點: (1) 逆向反應速率太快，導致開路電壓降低。 (2) 吸光係數太低，限制了在薄膜元件上效率的提升。所以，為了降低電子逆向傳遞的速率進而提高開路電壓，我們在RD5染料苯咪唑修飾甲基苯環上再導入高陰電性的氟原子；合成了一系列不同氟原子個數及不同位置的染料，進行深入的探討。RD12在甲基苯環的鄰、對位修飾上氟原子得到最佳的光電轉換效率9.5%，N719效率為9.3%。另外，我們利用瞬態光電流/光電壓衰減的量測技術，來探討開路電壓與氟原子個數及位置的關聯性，最後我們從元件的開路電壓與氟原子的個數得到了一個很好的關聯性。再進一步的延伸設計，為了改善RD系列染料低吸光係數的問題，我們將塞吩官能基導入；RD18染料含有二個塞吩取代基，成功的將吸光係數由RD12的8000提高到17000，而RD18也是這一系列異種釕金屬錯合物之中表現最佳的染料，短路電流為17.80 mAcm-2，開路電壓為0.735V，填充因子為0.73，光電轉換效率為9.6%，N719為8.8%。 近二年由於文獻中討論到這一系列含NSC-單芽基的染料，像是著名的N719染料，經過長時間照光，NSC-單芽基很容易被電解液中的添加劑所置換。為了提高染料的長效性，我們將二個NSC-單芽基以一環型雙芽基取代。在這個主題，我們成功合成了二個新型環型釕金屬錯合物RD22及RD26。並利用瞬態光電及電子萃取技術來分析及探討不同型態的染料於傳統碘液系統及新型態鈷錯合物系統電解液的一些逆向反應的機制。在鈷錯合物系統電解液中，RD26的光電轉換效率能達到7.17%，短路電流為11.779mAcm-2，開路電壓為0.817V，填充因子為0.745，而這個效率值為目前釕金屬錯合物在鈷錯合物系統電解液中的最高效率值。另外，在傳統碘液系統電解液的表現，也是目前這類型三邊修飾不同配位基的環型釕金屬錯合物中效率最高的記錄，光電轉換效率為9.19%，短路電流為17.144 mAcm-2，開路電壓為0.728V，填充因子為0.735。

The objective of this thesis is to develop a new class of ruthenium dyes for dye sensitized solar cells (DSSC) application. We designed a series of benzimidazole-based heteroleptic ruthenium complexs (RD series). Being different with the famous dyes based on bipyridine-ligand we generally known, for example: N719, Z907, and C101. First of all, we designed a novel ruthenium complex containing benzimidazole ligand. Benzimidazole, due to their electron mobilities are great, benzimidazole derivatives have been developed as layer materials for electron transport and hole blocking in organic light-emitting diode devices. RD1, the first prototype ruthenium dye in this study, with high dye loading ability, reaches 80 % cell performance of N719. Later, we decorated different substituents onto the imidazole ring, the result of benzimidazole-based dye contain benzyl group (RD5) had the best power conversion efficiency =7.7%, N719 (=7.8%). Compare to N719, RD series had some defects: (1) Back reaction rate was too fast, caused the Voc was reduced. (2) The extinction coefficient was lower that against thin film device development. Next, in order to reduce the back electron transfer rate to improve Voc value, RD5 was replaced with one fluorobenzyl-substituted ligand with fluorine atoms of varied number attached at various positions of the benzyl ring. RD12 which was designed two fluoro-substituted on benzyl group had the best performance =9.5% among this series compare to that of N719 (=9.3%). We used transient photocurrent/photovoltage decays method to measure the thin-film samples, and the experiment result has a good correlation to the effect of fluorine atoms on cell performance. In further study, we designed thiophene group to improve the extinction coefficient. The complex contain two thiophene groups (RD18) successfully increased extinction coefficient from 8000 to 17000, and it is the dye had the best power conversion efficiency among RD series dyes. Upon optimization, the device made of the RD18 dye gives Jsc/mA cm-2 = 17.80, Voc/V = 0.735, FF = 0.730, and η= 9.6 %, which is superior to the device made of N719 (η= 8.8 %) under the same fabrication conditions. The low stability of this series dyes with NCS- monodentate ligand such as N719 which was reported its desorption behavior from TiO2 surface and liberation of the NCS- ligands from the Ruthenium centre replacing by the additive of the electrolyte. To further improve the dye stability, replacing these labile monodentate ligands with chelatingcyclometalating ligands. In this topic, we successfully synthesized two novel cyclometalated ruthenium dyes RD22 and RD26. To analyze the different effects of the cobalt based electrolyte system and iodide/ tri-iodide based system with different type ruthenium dyes, we utilized the transient photoelectric and charge extraction measurements to explain how these different types of ruthenium dyes covered on TiO2 surface affect the electron on the TiO2 surface interacting with tri-iodide and Co(bpy)33+ and also used the photoinduced absorption and transient absorption spectroscopy techniques to explain how the different types of electrolyte affect the different types of ruthenium complexes. In cobalt based electrolyte system, to integrate the all the advantages of another three dyes, we got an outstanding performance of RD26, JSC/mAcm-2=11.779, VOC/V=0.817 and FF=0.745, with overall efficiency=7.17% of power conversion, which presents the highest efficiency in ruthenium complexes with cobalt based electrolyte system. In iodide/tri-iodide system, RD26 also performs the highest efficiency of trisheteroleptic cyclometalated ruthenium dyes with an overall efficiency of power conversion =9.19%, Jsc/mAcm-2=17.144, Voc/V=0.728, FF=0.735.